Most in ground planted cultivars tend to grow at or slightly below the soil line. This is particularly true in some of our heavy soils. The only concern that I would consider is how to prevent freezing in the cold months ahead. If you do not intend to dig your cannas you may need to mulch over the top well enough to prevent freezing.

The above issue may also be a result of not dividing in the spring and replanting below the soil line.